1. Field of the Invention
The present invention relates to aqueous phosphoric acid etching solutions used in the manufacture of semiconductor devices. More specifically, the invention provides an aqueous phosphoric acid etching solution that exhibits increased etch selectivity of silicon nitride films over silicon dioxide films in silicon nitride-silicon oxide composite semiconductor devices. The selective etching of silicon nitride over silicon dioxide is accomplished by use of an etch bath solution containing aqueous phosphoric acid and a silicon containing composition, such as hexafluorosilicic acid; that is readily soluble in the etch bath solution. For purposes of the invention "readily soluble" means that the silicon containing composition blends uniformly with the aqueous etch bath at room temperature. The etch bath solution does not contain hydrofluoric acid, nitric acid or mixtures thereof or compositions containing elements from Group I or II.
2. Description of Related Art
The fabrication of a composite semiconductor device consisting of a series of layers typically requires several basic processing steps. These basic processing steps include; cleaning, oxidation(atmospheric and high pressure), lithography(photolithography, electron-beam, and x-ray), doping by diffusion and implantation, deposition(electron-gun, sputter, chemical vapor deposition(CVD), low--pressure chemical vapor deposition(LPCVD), and plasma), etching(wet chemical and dry or plasma) and testing. These general techniques are well known to those skilled in the art.
The present invention provides an improved wet chemical etch method that increases the etch rate of silicon nitride and enhances the etch selectivity of a heated, aqueous phosphoric acid solution for silicon nitride relative to silicon dioxide in silicon nitride-silicon oxide composite semiconductor devices. This is accomplished by adding a readily soluble silicon containing composition, such as hexafuorosilicic acid to an aqueous, phosphoric acid containing etching bath solution. Unlike other wet etch methods using heated a phosphoric acid etch bath the silicon containing compositions of the present invention are readily soluble in the phosphoric acid containing etching bath solution. Furthermore, the solutions of the present invention do not contain hydrofluoric acid, nitric acid or mixtures thereof or compositions containing elements from Group I or II such as sodium metasilicate. The presence of sodium, magnesium and other Group I or II elements is undesirable in many semiconductor processing applications.
Phosphoric acid etching baths are known in the art. U.S. Pat. No. 4,092,211 to Northern Telecom Ltd. discloses a process for etching silicon nitride through apertures in a silicon dioxide mask in a boiling phosphoric acid bath. The process reduces the rate at which silicon dioxide is etched relative to the silicon nitride by adding a soluble silicate such as sodium metasilicate to the phosphoric acid or by adding a finely divided silica to the etching solution. According to the patent the addition of the soluble silicate or silica retards the etch rate of the silicon dioxide without affecting the etch rate of the silicon nitride. Japanese patent 60137024 to Matsushita Electric discloses the removal of a film formed from the by-products of etching a silicon nitride (Si.sub.3 N.sub.4) film that has previously been subjected to a heat treatment in the presence of moisture. The by-product residues, described in the patent by the formula Si.sub.x N.sub.y O.sub.z are removed by etching with a phosphoric acid solution containing fluorine ions. The object of the patent is to remove layers of silicon oxide, silicon nitride, and the by-product film of the silicon nitride heat treatment in an efficient manner. Japanese patent application JP 6349808 A to Hitachi Ltd., discloses a process of suppressing the etch rate of silicon dioxide film during silicon nitride film removal. This is accomplished according to the invention by using a phosphoric acid etching solution containing silicon. Ideally, the etching solution contains at least 50 ppm silicon. The level of the silicon in the etching bath is maintained by adding fresh etching solution from a make-up vessel. U.S. Pat. No. 5,310,457 to AT&T Bell Laboratories discloses a method of achieving high etch selectivity of both silicon nitride and silicon with respect to silicon dioxide by using an etch bath of phosphoric acid, hydrofluoric acid and nitric acid. According to the patent the addition of hydrofluoric acid and nitric acid in concentrations of less than 500 ppm, ". . . increases the polysilicon and silicon nitride etch rates by as much as several orders of magnitude and 100 percent respectively." The effectiveness of the etching bath is maintained by subsequent additions of hydrofluoric acid and nitric acid. U.S. Pat. No. 5,472,562 to AT&T Corporation discloses the addition of a silicon to an etch bath, comprising; phosphoric acid, hydrofluoric acid and nitric acid. The addition of the silicon to the etch bath is said to enhance the initial etch selectivity of silicon nitride or silicon with respect to silicon oxide. The silicon added to the etch bath is soluble in the bath. An example of a composition that is soluble is hexafluorosilicic acid.
None of the references disclose or suggest a phosphoric acid containing aqueous etch bath further containing composition containing silicon, such as hexafluorosilicic acid that is readily soluble in the aqueous, phosphoric etch bath and the selective etching capabilities of this bath. The use of silicates such as those disclosed in U.S. Pat. No. 4,092,211 are not contemplated as part of the present invention.